Absolute Configuration of Periplosides C and F and Isolation of Minor Spiro-orthoester Group-Containing Pregnane-type Steroidal Glycosides from <i>Periploca sepium</i> and Their T‑Lymphocyte Proliferation Inhibitory Activities

Further phytochemical investigation of the root bark of <i>Periploca sepium</i> afforded nine new spiro-orthoester group-containing pregnane-type glycosides termed periplosides O–V and 3-<i>O</i>-formyl-periploside A. The structures of these glycosides along with the absolute configuration of the unique seven-membered formyl acetal-bridged spiro-orthoester function and the 4,6-dideoxy-3-<i>O</i>-methyl-Δ³-2-hexosulosyl moiety were elucidated on the basis of spectroscopic data interpretation and chemical transformation. The absolute configurations of the major compounds periplosides C and F were established by single-crystal X-ray diffraction analysis. The isolated compounds were evaluated for their inhibitory activities against the proliferation of T-lymphocytes. As a result, periploside C, the most abundant glycoside containing a spiro-orthoester moiety found in the plant, exhibited the most favorite selective index value (SI = 82.5). The length and constitution of the saccharide chain in the periplosides were found to influence the inhibitory activity and the SI value